SSHD(1M)SSHD(1M)NAMEsshd - secure shell daemon
SYNOPSISsshd [-deiqtD46] [-b bits] [-f config_file]
[-g login_grace_time] [-h host_key_file]
[-k key_gen_time] [-p port] [-V client_protocol_id]
DESCRIPTION
The sshd (Secure Shell daemon) is the daemon program for ssh(1).
Together these programs replace rlogin and rsh, and provide secure
encrypted communications between two untrusted hosts over an insecure
network. The programs are intended to be as easy to install and use as
possible.
sshd is the daemon that listens for connections from clients. It forks
a new daemon for each incoming connection. The forked daemons handle
key exchange, encryption, authentication, command execution, and data
exchange.
This implementation of sshd supports both SSH protocol versions 1 and 2
simultaneously. Because of security weaknesses in the v1 protocol,
sites should run only v2, if possible. In the default configuration,
only protocol v2 is enabled for the server. To enable v1 and v2 simul‐
taneously, see the instructions in sshd_config(4).
Support for v1 is provided to help sites with existing ssh v1 clients
and servers to transition to v2. v1 might not be supported in a future
release.
SSH Protocol Version 1
Each host has a host-specific RSA key (normally 1024 bits) used to
identify the host. Additionally, when the daemon starts, it generates a
server RSA key (normally 768 bits). This key is normally regenerated
every hour if it has been used, and is never stored on disk.
Whenever a client connects the daemon responds with its public host and
server keys. The client compares the RSA host key against its own data‐
base to verify that it has not changed. The client then generates a
256-bit random number. It encrypts this random number using both the
host key and the server key, and sends the encrypted number to the
server. Both sides then use this random number as a session key which
is used to encrypt all further communications in the session. The rest
of the session is encrypted using a conventional cipher, currently
Blowfish or 3DES, with 3DES being used by default. The client selects
the encryption algorithm to use from those offered by the server.
Next, the server and the client enter an authentication dialog. The
client tries to authenticate itself using .rhosts authentication,
.rhosts authentication combined with RSA host authentication, RSA chal‐
lenge-response authentication, or password-based authentication.
Rhosts authentication is normally disabled because it is fundamentally
insecure, but can be enabled in the server configuration file if
desired. System security is not improved unless rshd(1M), rlogind(1M),
rexecd(1M), and rexd(1M) are disabled (thus completely disabling
rlogin(1) and rsh(1) into the machine).
SSH Protocol Version 2
Version 2 works similarly to version 1: Each host has a host-specific
DSA/RSA key. However, when the daemon starts, it does not generate a
server key. Forward security is provided through a Diffie-Hellman key
agreement. This key agreement results in a shared session key. The rest
of the session is encrypted using a symmetric cipher, currently 128-bit
AES, Blowfish, 3DES, or AES. The client selects the encryption algo‐
rithm to use from those offered by the server. Additionally, session
integrity is provided through a cryptographic message authentication
code (hmac-sha1 or hmac-md5).
Protocol version 2 provides a public key based user authentication
method (PubKeyAuthentication) GSS-API based user authentication, con‐
ventional password authentication, and a generic prompt/reply protocol
for password-based authentication.
Command Execution and Data Forwarding
If the client successfully authenticates itself, a dialog for preparing
the session is entered. At this time the client can request things like
allocating a pseudo-tty, forwarding X11 connections, forwarding TCP/IP
connections, or forwarding the authentication agent connection over the
secure channel.
Finally, the client either requests a shell or execution of a command.
The sides then enter session mode. In this mode, either side may send
data at any time, and such data is forwarded to/from the shell or com‐
mand on the server side, and the user terminal on the client side.
When the user program terminates and all forwarded X11 and other con‐
nections have been closed, the server sends command exit status to the
client, and both sides exit.
sshd can be configured using command-line options or the configuration
file /etc/ssh/ssh_config, described in ssh_config(4). Command-line
options override values specified in the configuration file.
sshd rereads its configuration file when it receives a hangup signal,
SIGHUP, by executing itself with the name it was started as, that is,
/usr/lib/ssh/sshd.
Host Access Control
The sshd daemon uses TCP Wrappers to restrict access to hosts. It uses
the service name of sshd for hosts_access(). For more information on
TCP Wrappers see tcpd(1M) and hosts_access(3) man pages, which are part
of the SUNWsfman package (they are not SunOS man pages). TCP wrappers
binaries, including libwrap, are in SUNWtcpd, a required package for
SUNWsshdu, the package containing sshd.
OPTIONS
The options for sshd are as follows:
-b bits
Specifies the number of bits in the server key (the default is
768).
-d
Debug mode. The server sends verbose debug output to the system
log, and does not put itself in the background. The server also
will not fork and will only process one connection. This option is
only intended for debugging for the server. Multiple -d options
increase the debugging level. Maximum is 3.
-e
When this option is specified, sshd will send the output to stan‐
dard error instead of to the system log.
-f configuration_file
Specifies the name of the configuration file. The default is
/etc/ssh/sshd_config. sshd refuses to start if there is no configu‐
ration file.
-g login_grace_time
Gives the grace time for clients to authenticate themselves (the
default is 300 seconds). If the client fails to authenticate the
user within this number of seconds, the server disconnects and
exits. A value of zero indicates no limit.
-h host_key_file
Specifies a file from which a host key is read. This option must be
given if sshd is not run as root (as the normal host key files are
normally not readable by anyone but root). The default is
/etc/ssh/ssh_host_key for protocol version 1, and
/etc/ssh/ssh_host_rsa_key and /etc/ssh/ssh_host_dsa_key for proto‐
col version 2. It is possible to have multiple host key files for
the different protocol versions and host key algorithms.
-i
Specifies that sshd is being run from inetd. sshd is normally not
run from inetd because it needs to generate the server key before
it can respond to the client, and this may take tens of seconds.
Clients would have to wait too long if the key was regenerated
every time. However, with small key sizes (for example, 512) using
sshd from inetd may be reasonable.
-k key_gen_time
(SSHv1-specific) Specifies how often the server key is regenerated
(the default is 3600 seconds, or one hour). The motivation for
regenerating the key fairly often is that the key is not stored
anywhere, and after about an hour, it becomes impossible to recover
the key for decrypting intercepted communications even if the
machine is cracked into or physically seized. A value of zero indi‐
cates that the key will never be regenerated.
-o option
Can be used to specify options in the format used in the configura‐
tion file. This is useful for specifying options for which there
are no separate command-line flags.
-p port
Specifies the port on which the server listens for connections (the
default is 22).
-q
Quiet mode. Nothing is sent to the system log. Normally the begin‐
ning, authentication, and termination of each connection is logged.
-t
Test mode. Check only the validity of the configuration file and
the sanity of the keys. This is useful for updating sshd reliably
as configuration options might change.
-D
When this option is specified sshd does not detach and does not
become a daemon. This allows easy monitoring of sshd.
-4
Forces sshd to use IPv4 addresses only.
-6
Forces sshd to use IPv6 addresses only.
EXTENDED DESCRIPTION
authorized_keys File Format
The $HOME/.ssh/authorized_keys file lists the public keys that are per‐
mitted for RSA authentication in protocol version 1 and for public key
authentication (PubkeyAuthentication) in protocol version 2. The Autho‐
rizedKeysFile configuration option can be used to specify an alterna‐
tive file.
Each line of the file contains one key (empty lines and lines starting
with a hash mark [#] are ignored as comments).
For each RSA key for protocol version 1, the file consists of the fol‐
lowing space-separated fields:
options bits exponent modulus comment
For the public key for protocol version 2, the file consists of the
following space-separated fields:
options key-type base64-encoding-key comment
For protocol version 2, key-type is one of ssh-rsa or ssh-dsa.
The options field is optional; its presence is determined by whether
the line starts with a number. (The option field never starts with a
number.) The bits, exponent, and modulus fields give the RSA key; the
comment field is a convenient place for you to identify the key.
Lines in this file are usually several hundred bytes long (because of
the size of the key modulus). You will find it very inconvenient to
type them in; instead, copy the public key file and edit it.
Permissions of this file must be set so that it is not world or group
writable. See the StrictModes option of sshd_config(4).
The options (if present) consist of comma-separated option specifica‐
tions. No spaces are permitted, except within double quotes. The fol‐
lowing option specifications are supported:
from="pattern-list"
Specifies that, in addition to public key authentication, the
canonical name of the remote host must be present in the comma-sep‐
arated list of patterns (`*' and `?' serve as wildcards). The list
can also contain negated patterns by prefixing the patterns with
`!'. If the canonical host name matches a negated pattern, the key
is not accepted.
The purpose of this option is to give you the option of increasing
security: public key authentication by itself does not trust the
network or name servers or anything but the key. However, if some‐
one manages to steal the key, possession of the key would permit
the intruder to log in from anywhere in the world. This option
makes using a stolen key more difficult, because name servers and
routers would have to be compromised, in addition to just the key.
command="command"
Specifies that the command is executed whenever this key is used
for authentication. The command supplied by the user (if any) is
ignored. The command is run on a pty if the client requests a pty;
otherwise it is run without a tty. If an 8-bit clean channel is
required, one must not request a pty or should specify no-pty. You
can include a quote in the command by escaping it with a backslash.
This option might be useful to restrict certain public keys from
performing a specific operation. An example is a key that permits
remote backups but nothing else. Note that the client can specify
TCP/IP and/or X11 forwarding unless they are explicitly prohibited
from doing so. Also note that this option applies to shell, com‐
mand, or subsystem execution.
environment="NAME=value"
Specifies that the string NAME=value is to be added to the environ‐
ment when logging in using this key. Environment variables set this
way override other default environment values. Multiple options of
this type are permitted. Environment processing is disabled by
default and is controlled via the PermitUserEnvironment option.
no-port-forwarding
Forbids TCP/IP forwarding when this key is used for authentication.
Any port forward requests by the client will return an error. This
might be used, for example, in connection with the command option.
no-X11-forwarding
Forbids X11 forwarding when this key is used for authentication.
Any X11 forward requests by the client will return an error.
no-agent-forwarding
Forbids authentication agent forwarding when this key is used for
authentication.
no-pty
Prevents tty allocation (a request to allocate a pty will fail).
permitopen="host:port"
Limit local ssh -L port forwarding such that it can connect only to
the specified host and port. IPv6 addresses can be specified with
an alternative syntax: host/port. You can invoke multiple permi‐
topen options, with each instance separated by a comma. No pattern
matching is performed on the specified hostnames. They must be lit‐
eral domains or addresses.
ssh_known_hosts File Format
The /etc/ssh/ssh_known_hosts and $HOME/.ssh/known_hosts files contain
host public keys for all known hosts. The global file should be pre‐
pared by the administrator (optional), and the per-user file is main‐
tained automatically: whenever the user connects from an unknown host
its key is added to the per-user file.
For the RSA key for protocol version 1, these files consist of the fol‐
lowing space-separated fields:
hostnames bits exponent modulus comment
For the public key for protocol version 2, these files consist of the
following space-separated fields:
hostnames key-type base64-encoding-key comment
For protocol version 2, key-type is one of ssh-rsa or ssh-dsa.
Hostnames is a comma-separated list of patterns (* and ? act as wild‐
cards); each pattern in turn is matched against the canonical host name
(when authenticating a client) or against the user-supplied name (when
authenticating a server). A pattern can also be preceded by ! to indi‐
cate negation: if the host name matches a negated pattern, it is not
accepted (by that line) even if it matched another pattern on the line.
Alternately, hostnames can be stored in a hashed form, which hides host
names and addresses should the file's contents be disclosed. Hashed
hostnames start with a vertical bar (|) character. Only one hashed
hostname can appear on a single line and none of the above negation or
wildcard operators may be applied.
Bits, exponent, and modulus are taken directly from the RSA host key;
they can be obtained, for example, from /etc/ssh/ssh_host_rsa_key.pub.
The optional comment field continues to the end of the line, and is not
used.
Lines starting with a hash mark (#) and empty lines are ignored as com‐
ments.
When performing host authentication, authentication is accepted if any
matching line has the proper key. It is thus permissible (but not rec‐
ommended) to have several lines or different host keys for the same
names. This will inevitably happen when short forms of host names from
different domains are put in the file. It is possible that the files
contain conflicting information; authentication is accepted if valid
information can be found from either file.
The lines in these files are typically hundreds of characters long. You
should definitely not type in the host keys by hand. Rather, generate
them by a script or by taking /etc/ssh/ssh_host_rsa_key.pub and adding
the host names at the front.
ENVIRONMENT VARIABLESsshd sets the following environment variables for commands executed by
ssh users:
DISPLAY
Indicates the location of the X11 server. It is automatically set
by sshd to point to a value of the form hostname:n, where hostname
indicates the host where the shell runs, and n is an integer
greater than or equal to 1. ssh uses this special value to forward
X11 connections over the secure channel. Unless you have important
reasons to do otherwise, you should not set DISPLAY explicitly, as
that will render the X11 connection insecure and will require you
to manually copy any required authorization cookies.
HOME
Set to the path of the user's home directory.
LANG, LC_ALL, LC_COLLATE, LC_CTYPE, LC_MESSAGES, LC_MONETARY,
LC_NUMERIC, LC_TIME
A locale setting. The locale defaults to that of sshd (usually the
system-wide default locale), or is negotiated between the client
and server during initial key exchange (as per RFC 4253).
Following initial key exchange, each of the variables can be over‐
riden in the following sequence:
1. If a locale setting is set in a client's environment and
that client supports "Environment Variable Passing" (see
RFC 4254), then the setting will be passed over to the
server side.
2. If the public key authentication method was used to
authenticate the server and the PermitUserEnvironment
variable in sshd_config(4) is set to yes on the server
side, then the setting can be changed through the use of
the environment option in the client's Authorized‐
KeysFile file.
3. The setting can be change in the client's ~/.ssh/envi‐
ronment file on the server.
See PermitUserEnvironment in sshd_config(4) as to when the Autho‐
rizedKeysFile and ~/.ssh/environment files are processed and used
for setting the user environment.
LOGNAME
Synonym for USER. Set for compatibility with systems that use this
variable.
MAIL
Set to point to the user's mailbox.
SSH_AUTH_SOCK
Indicates the path of a unix-domain socket used to communicate with
the agent.
SSH_CONNECTION
Identifies the client and server ends of the connection. The vari‐
able contains four space-separated values: client IP address,
client port number, server IP address and server port number.
SSH_CLIENT
Identifies the client end of the connection. The variable contains
three space-separated values: client IP address, client port num‐
ber, and server port number.
SSH_TTY
Set to the name of the tty (path to the device) associated with the
current shell or command. If the current session has no tty, this
variable is not set.
TZ
Indicates the present timezone, if TIMEZONE is set in
/etc/default/login or if TZ was set when the daemon was started.
HZ
If set in /etc/default/login, the daemon sets it to the same value.
SHELL
The user's shell, if ALTSHELL=YES in /etc/default/login.
PATH
Set to the value of PATH or SUPATH (see login(1)) in
/etc/default/login, or, if not set, to /usr/bin:/bin.
USER
Set to the name of the user logging in.
Additionally, sshd reads $HOME/.ssh/environment and adds lines of the
format VARNAME=value to the environment.
EXAMPLES
In the following examples, certain lines might wrap due to line length
limits in your display. You should nevertheless consider the wrapped
line as a single line.
Example 1 authorized_key File Entries
The following are examples of authorized_key file entries for protocol
1:
1024 33 12121...312314325 ylo@foo.bar
from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23...2334 ylo@niksula
command="dump /home",no-pty,no-port-forwarding 1024 33 23...2323 backup.hut.fi
Example 2 authorized_key File Entries for Protocol 2
The following are examples of authorized_key file entries for protocol
2:
ssh-rsa AAAAB3NzaC1y.....EU88ovYKg4GfclWGCFYTuw8= ylo@foo.bar
from="*.niksula.hut.fi" ssh-rsa AAAAB3NzaC...uw8= ylo@niksula
command="dump /home",no-pty,no-port-forwarding ssh-rsa AA..8= backup.hut.fi
Example 3 ssh_known_hosts File Entries for Protocol 1
The following are examples of ssh_known_hosts file entries for protocol
1:
closenet,closenet.hut.fi,...,130.233.208.41 1024 37 159...93 closenet.hut.fi
Example 4 ssh_known_hosts File Entries for Protocol 2
The following are examples of ssh_known_hosts file entries for protocol
2:
closenet,closenet.hut.fi,...,130.233.208.41 ssh-rsa AA..8= closenet.hut.fi
EXIT STATUS
The following exit values are returned:
0
Successful completion.
>0
An error occurred.
FILES
/etc/default/login
Contains defaults for several sshd_config parameters, environment
variables, and other environmental factors.
The following parameters affect environment variables (see login(1)
and descriptions of these variables, above):
o TIMEZONE
o HZ
o ALTSHELL
o PATH
o SUPATH
The following /etc/default/login parameters supply default values
for corresponding sshd_config(4) parameters:
o CONSOLE (see PermitRootLogin in sshd_config(4))
o PASSREQ (see PermitEmptyPasswords in sshd_config(4))
o TIMEOUT (see LoginGraceTime in sshd_config(4))
The following /etc/default/login parameters:
o UMASK
o ULIMIT
...set the umask(2) and file size limit of, respectively, the
shells and commands spawned by sshd.
Finally, two /etc/default/login parameters affect the maximum
allowed login attempts per-connection using interactive user
authentication methods (for example, keyboard-interactive but not
publickey), as per login(1):
o RETRIES
o SYSLOG_FAILED_LOGINS
/etc/ssh/sshd_config
Contains configuration data for sshd. This file should be writable
by root only, but it is recommended (though not necessary) that it
be world-readable.
/etc/ssh/ssh_host_key
/etc/ssh/ssh_host_dsa_key
/etc/ssh/ssh_host_rsa_key
Contains the private part of the host key. This file should only be
owned by root, readable only by root, and not accessible to others.
sshd does not start if this file is group/world-accessible.
/etc/ssh/ssh_host_key.pub
/etc/ssh/ssh_host_dsa_key.pub
/etc/ssh/ssh_host_rsa_key.pub
Contains the public part of the host key. This file should be
world-readable but writable only by root. Its contents should match
the private part. This file is not used for encryption; it is pro‐
vided only for the convenience of the user so its contents can be
copied to known hosts files. These two files are created using ssh-
keygen(1).
/var/run/sshd.pid
Contains the process ID of the sshd listening for connections. If
there are several daemons running concurrently for different ports,
this contains the pid of the one started last. The content of this
file is not sensitive; it can be world-readable. You can use the
PidFile keyword in sshd_config to specify a file other than
/var/run/sshd.pid. See sshd_config(4).
/etc/ssh/ssh_known_hosts and $HOME/.ssh/known_hosts
These files are consulted when using rhosts with public key host
authentication to check the public key of the host. The key must be
listed in one of these files to be accepted. The client uses the
same files to verify that the remote host is the one it intended to
connect. These files should be writable only by root or the owner.
/etc/ssh/ssh_known_hosts should be world-readable, and
$HOME/.ssh/known_hosts can but need not be world-readable.
/etc/nologin
If this file exists, sshd refuses to let anyone except root log in.
The contents of the file are displayed to anyone trying to log in,
and non-root connections are refused. The file should be world-
readable.
$HOME/.ssh/authorized_keys
Lists the public keys (RSA or DSA) that can be used to log into the
user's account. This file must be readable by root. This might, on
some machines, imply that it is world-readable if the user's home
directory resides on an NFS volume. It is recommended that it not
be accessible by others. The format of this file is described
above. Users will place the contents of their identity.pub,
id_dsa.pub and/or id_rsa.pub files into this file, as described in
ssh-keygen(1).
$HOME/.rhosts
This file contains host-username pairs, separated by a space, one
per line. The given user on the corresponding host is permitted to
log in without password. The same file is used by rlogind and
rshd. The file must be writable only by the user; it is recommended
that it not be accessible by others. It is also possible to use
netgroups in the file. Either host or user name may be of the form
+@groupname to specify all hosts or all users in the group.
$HOME/.shosts
For ssh, this file is exactly the same as for .rhosts. However,
this file is not used by rlogin and rshd, so using this permits
access using SSH only.
/etc/hosts.equiv
This file is used during .rhosts authentication. In its simplest
form, this file contains host names, one per line. Users on these
hosts are permitted to log in without a password, provided they
have the same user name on both machines. The host name can also be
followed by a user name; such users are permitted to log in as any
user on this machine (except root). Additionally, the syntax
+@group can be used to specify netgroups. Negated entries start
with a hyphen (-).
If the client host/user is successfully matched in this file, login
is automatically permitted, provided the client and server user
names are the same. Additionally, successful RSA host authentica‐
tion is normally required. This file must be writable only by
root; it is recommended that it be world-readable.
Warning: It is almost never a good idea to use user names in
hosts.equiv. Beware that it really means that the named user(s)
can log in as anybody, which includes bin, daemon, adm, and other
accounts that own critical binaries and directories. For practical
purposes, using a user name grants the user root access. Probably
the only valid use for user names is in negative entries. This
warning also applies to rsh/rlogin.
/etc/ssh/moduli
A private file.
/etc/ssh/shosts.equiv
This file is processed exactly as /etc/hosts.equiv. However, this
file might be useful in environments that want to run both
rsh/rlogin and ssh.
$HOME/.ssh/environment
This file is read into the environment at login (if it exists). It
can contain only empty lines, comment lines (that start with #),
and assignment lines of the form name=value. The file should be
writable only by the user; it need not be readable by anyone else.
Environment processing is disabled by default and is controlled by
means of the PermitUserEnvironment option.
$HOME/.ssh/rc
If this file exists, it is run with /bin/sh after reading the envi‐
ronment files but before starting the user's shell or command. If
X11 spoofing is in use, this will receive the proto cookie pair in
standard input (and DISPLAY in environment). This must call xauth
in that case.
The primary purpose of $HOME/.ssh/rc is to run any initialization
routines that might be needed before the user's home directory
becomes accessible; AFS is a particular example of such an environ‐
ment. If this file exists, it is run with /bin/sh after reading the
environment files, but before starting the user's shell or command.
It must not produce any output on stdout; stderr must be used
instead. If X11 forwarding is in use, it will receive the proto
cookie pair in its standard input and DISPLAY in its environment.
The script must call xauth because sshd will not run xauth automat‐
ically to add X11 cookies.
This file will probably contain some initialization code followed
by something similar to:
if read proto cookie && [ -n "$DISPLAY" ]
then
if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]
then
# X11UseLocalhost=yes
echo add unix:`echo $DISPLAY |
cut -c11-` $proto $cookie
else
# X11UseLocalhost=no
echo add $DISPLAY $proto $cookie
fi | xauth -q -
fi
If this file does not exist, /etc/ssh/sshrc is run, and if that
does not exist, xauth is used to store the cookie. $HOME/.ssh/rc
should be writable only by the user, and need not be readable by
anyone else.
/etc/ssh/sshrc
Similar to $HOME/.ssh/rc. This can be used to specify machine-spe‐
cific login-time initializations globally. This file should be
writable only by root, and should be world-readable.
SECURITYsshd supports the use of several user authentication mechanisms: a pub‐
lic key system where keys are associated with users (through users'
authorized_keys files), a public key system where keys are associated
with hosts (see the HostbasedAuthentication configuration parameter), a
GSS-API based method (see the GssAuthentication and GssKeyEx configura‐
tion parameters) and three initial authentication methods: none, pass‐
word, and a generic prompt/reply protocol, keyboard-interactive.
sshd negotiates the use of the GSS-API with clients only if it has a
GSS-API acceptor credential for the "host" service. This means that,
for GSS-API based authentication, the server must have a Kerberos V
keytab entry (see below) or the equivalent for any other GSS-API mecha‐
nism that might be installed.
In order for Kerberos authentication to work, a host/<FQDN> Kerberos
principal must exist for each Fully Qualified Domain Name associated
with the in.sshd server. Each of these host/<FQDN> principals must have
a keytab entry in the /etc/krb5/krb5.keytab file on the in.sshd server.
An example principal might be:
host/bigmachine.eng.example.com
See kadmin(1M) or gkadmin(1M) for instructions on adding a principal to
a krb5.keytab file. See for a discussion of Kerberos authentication.
GSS-API authorization is covered in gss_auth_rules(5).
sshd uses pam(3PAM) for the three initial authentication methods as
well as for account management, session management, and password man‐
agement for all authentication methods.
Specifically, sshd calls pam_authenticate() for the "none," "password"
and "keyboard-interactive" SSHv2 userauth types, as well as for for the
null and password authentication methods for SSHv1. Other SSHv2 authen‐
tication methods do not call pam_authenticate(). pam_acct_mgmt() is
called for each authentication method that succeeds.
pam_setcred() and pam_open_session() are called when authentication
succeeds and pam_close_session() is called when connections are closed.
pam_open_session() and pam_close_session() are also called when SSHv2
channels with ptys are opened and closed.
Each SSHv2 userauth type has its own PAM service name:
┌─────────────────────┬──────────────────┐
│ SSHv2 Userauth │ PAM Service Name │
├─────────────────────┼──────────────────┤
│none │ sshd-none │
├─────────────────────┼──────────────────┤
│password │ sshd-password │
├─────────────────────┼──────────────────┤
│keyboard-interactive │ sshd-kbdint │
├─────────────────────┼──────────────────┤
│pubkey │ sshd-pubkey │
├─────────────────────┼──────────────────┤
│hostbased │ sshd-hostbased │
├─────────────────────┼──────────────────┤
│gssapi-with-mic │ sshd-gssapi │
├─────────────────────┼──────────────────┤
│gssapi-keyex │ sshd-gssapi │
└─────────────────────┴──────────────────┘
For SSHv1, sshd-v1 is always used.
If pam_acct_mgmt() returns PAM_NEW_AUTHTOK_REQD (indicating that the
user's authentication tokens have expired), then sshd forces the use of
"keyboard-interactive" userauth, if version 2 of the protocol is in
use. The "keyboard-interactive" userauth will call pam_chauthtok() if
pam_acct_mgmt() once again returns PAM_NEW_AUTHTOK_REQD. By this means,
administrators are able to control what authentication methods are
allowed for SSHv2 on a per-user basis.
Setting up Host-based Authentication
To establish host-based authentication, you must perform the following
steps:
o Configure the client.
o Configure the server.
o Publish known hosts.
o Make appropriate entries in /etc/ssh/shosts.equiv and
~/.shosts.
These steps are expanded in the following paragraphs.
o On a client machine, in the system-wide client configuration
file, /etc/ssh/ssh_config, you must have the entry:
HostbasedAuthentication yes
See ssh_config(4) and ssh-keysign(1M).
o On the server, in the system-wide server configuration file,
/etc/ssh/sshd_config, you must have the entry:
HostbasedAuthentication yes
If per-user .shost files are to be allowed (see last step),
in the same file, you must have:
IgnoreRhosts no
See sshd_config(4) for a description of these keywords.
o To publish known hosts, you must have entries for the
clients from which users will be allowed host-based authen‐
tication. Make these entries in either or both of the sys‐
tem-wide file (/etc/ssh/ssh_known_hosts) or the per-user
file (~/.ssh/known_hosts).
o Note that sshd uses .shosts, not .rhosts. If you want the
functionality provided by .rhosts, but do not want to use
rlogin or rsh because of their security shortcomings, you
can use .shosts in conjunction with sshd. To use this fea‐
ture, make appropriate entries in /etc/ssh/shosts.equiv and
~/.shosts, in the format specified in rhosts(4).
For the vast majority of network environments, .shosts is
preferred over .rhosts.
ATTRIBUTES
See attributes(5) for descriptions of the following attributes:
┌────────────────────┬─────────────────┐
│ ATTRIBUTE TYPE │ ATTRIBUTE VALUE │
├────────────────────┼─────────────────┤
│Interface Stability │ Evolving │
└────────────────────┴─────────────────┘
The interface stability of /etc/ssh/moduli is Private.
SEE ALSOlogin(1), scp(1), ssh(1), ssh-add(1), ssh-agent(1), ssh-keygen(1),
svcs(1), gkadmin(1M), kadmin(1M), sftp-server(1M), ssh-keysign(1M),
svcadm(1M), pam(3PAM), rhosts(4), ssh_config(4), sshd_config(4),
attributes(5), gss_auth_rules(5), kerberos(5), pam_roles(5), smf(5)NOTES
The sshd service is managed by the service management facility, smf(5),
under the service identifier:
svc:/network/ssh:default
Administrative actions on this service, such as enabling, disabling, or
requesting restart, can be performed using svcadm(1M). The service's
status can be queried using the svcs(1) command.
sshd always sets PAM_RHOST and sets PAM_AUSER in the case of host-based
userauth. This behavior allows for remote logins to roles using host-
based authentication. See pam_roles(5).
Mar 17, 2009 SSHD(1M)
